1. Field of the Invention
This invention relates to a process for combustion of a gaseous and/or liquid fuel which has the potential for increasing thermal efficiency and reducing NO.sub.x emissions from conventional heating apparatuses such as boilers and other fluid heaters, and which makes possible the use of boilers and other fluid heaters having a reduced size in comparison to conventional boilers and fluid heaters having a comparable thermal rating. More particularly, this invention relates to heating apparatuses which employ porous matrix beds similar to those used in porous matrix surface combustors, differing from said surface combustors in that the flame normally present in the surface combustor is removed to a free space upstream of the porous matrix bed, which allows conventional flame detection and control and facilitates turndown.
2. Description of Prior Art
In conventional types of fluid heaters, the combustion chamber occupies most of the volume of the heating apparatus, thereby undesirably affecting the performance and the cost of the heating apparatus. Accordingly, it is desirable to develop a small but highly efficient fluid heater. U.S. Pat. No. 5,482,009 teaches two methods for reducing the volume of a boiler occupied by a furnace to nearly zero.
The first of these methods is a high intensity surface combustion method which attains high intensity surface combustion by use of a premixed burner and a tube nested combustion chamber in which the combustion and heat transfer are attained by causing the flame from the burner to penetrate into a nest of water tubes. A compact boiler comprising a water tube-walled combustion chamber and a forced draft convective heat exchange section disposed downstream of the combustion chamber comprising a helical coil assembly is taught by U.S. Pat. No. 5,259,342. Combustion is carried out in stages and, in accordance with at least one embodiment, flue gas recirculation is employed wherein the recirculated flue gases are introduced into the burner combustion air.
The reduction of NO.sub.x emissions from combustion processes and apparatuses is a long-standing, major environmental objective. As a result, numerous strategies for reducing the NO.sub.x emissions from combustion processes have been developed. One such strategy is the use of staged combustion in which the fuel or combustion air is introduced into the combustion apparatus in two or more stages, thereby maintaining flame temperature below a threshold temperature required for significant formation of NO.sub.x in the combustion process. U.S. Pat. No. 5,080,577 teaches a method and apparatus for staged combustion within porous matrix elements in which a fuel and source of oxygen are mixed and combusted in at least two successive combustion zones filled with a porous matrix. The first combustion zone is fuel-rich and the second combustion zone is fuel-lean. The use of fuel-rich conditions in the first stage of a staged combustion method and apparatus reduces the potential for NO.sub.x formation by ensuring that there is substantially no free oxygen available to combine with nitrogen which may be present in the first combustion stage and by maintaining a temperature in the first stage of the combustion process below the temperature at which NO.sub.x is formed, typically about 1800.degree. F. U.S. Pat. No. 4,878,830 teaches a staged combustion method and apparatus in which a fuel is burned in the first stage under fuel-rich conditions and the temperature of the products of combustion are controlled by a heat transfer device disposed between the primary and secondary combustion zones downstream of which secondary combustion oxygen is introduced into the apparatus for completing combustion. Similarly, U.S. Pat. No. 5,645,413 teaches a method and apparatus for low NO.sub.x combustion in natural draft combustion chambers comprising a staged-air combustion apparatus and process in which a fuel-rich, fuel-air mixture is introduced through a burner into a combustion chamber, forming a fuel-rich primary combustion zone flame which is attached to a flame guide disposed substantially parallel to the longitudinal axis of the primary combustion zone flame. The flame guide provides for removal of heat from the primary combustion zone flame. Secondary combustion air is introduced into the combustion chamber downstream of the burner to complete the combustion of the fuel-rich primary combustion zone flame products. To stabilize the fuel-rich combustion zone flame, combustion gases are internally recirculated within the combustion chamber into the fuel-rich primary combustion zone flame. U.S. Pat. No. 5,709,541 teaches a staged combustion process in which the first stage is fuel-rich, secondary combustion air is introduced downstream of the primary combustion zone and the combustion gases are internally recirculated into the fuel-rich primary combustion zone. Heat is removed from the combustion process by means of process tubes disposed within the combustion zones. In U.S. Pat. No. 5,052,921, a gas-fired burner is operated at substantially sub-stoichiometric conditions to produce products of combustion rich in combustibles and control flame temperatures at temperatures which do not exceed predetermined levels. Completion air is subsequently employed to complete combustion. Regenerative and recuperative means are used to cool gases after each partial combustion step.
Yet another method for controlling NO.sub.x emission is the use of recirculated flue gases. U.S. Pat. No. 5,044,932 teaches a process and apparatus for reducing NO.sub.x content of the flue gas effluent from a furnace comprising a burner assembly having a burner and flue gas recirculating system for collecting and passing internally recirculated flue gas into a combustion zone for reaction with a combustion flame and comprising a wall tube system for removal of heat.
Staged combustion in combination with removal of heat by heat exchange means is taught by U.S. Pat. No. 4,395,223 wherein both the primary and secondary combustion zones are fuel-lean, U.S. Pat. No. 4,013,399 which teaches burning fuel in a primary combustion chamber under fuel-rich conditions and passing the combustion products through a secondary combustion zone into which more air is injected wherein the combustion zones are surrounded by a group of water filled tubes, and U.S. Pat. No. 4,867,674 which teaches a method in which a process medium is heated in two stages wherein fuel oil and gas premixed with air are partially burned in a primary combustion chamber under fuel-rich conditions downstream of which additional combustion air is mixed with the combustion products from the primary combustion chamber to complete combustion of the unburned portions.